The invention relates to a transformerless direct current-direct current (DC/DC) converter, which is operated in the resonant mode of operation, has boosting properties and affords grounding of the DC source.
Transformerless inverters, which allow for boosting a direct-current (DC) source voltage, are known from prior art. The document DE 10 2004 037 446 B4 describes such an inverter.
A symmetrical boosting switch is connected downstream of the DC source. It is followed by a direct current-alternating current (DC-AC) converter in the form of a single- or multiple-phase bridge circuit. The boost converter allows for boosting the DC input voltage as large as necessary. On photovoltaic inverters, the input voltage at the DC-AC converter must be higher than the grid peak voltage in order to feed the grid. If the DC source voltage is not high enough, a boost converter is suited to boost the voltage to the value required. If the DC source voltage is high enough, the boost converter is inactive. A disadvantage is the conductive losses of the boost converter diodes, losses which also occur when the boost converter is not active. These diodes always lie in the current path between the source and the DC-AC converter. Another disadvantage of the prior art mentioned is that there is no possibility of grounding the source.
On DC/DC converters comprising only one voltage input with only one positive pole and one negative pole, which do not contain any additional grounding terminal, either the negative pole or the positive pole must be grounded if grounding takes place on the input side. It is desired to ground the negative pole or the positive pole on photovoltaic modules of a photovoltaic plant for example. An advantage is thereby obtained with converter circuits that allow for grounding at the voltage input. A particular advantage is obtained if the converter circuit affords grounding at any pole.
The grounding of the DC source, in particular of the photovoltaic module of a photovoltaic plant, eliminates the problem of high capacitive leakage currents occurring on transformerless inverters in photovoltaic plants. These leakage currents are to be reduced or, even better, to be avoided. Transformerless solutions however are preferred because of their light weight and of the cost advantages.
Moreover, the input voltage varies very strongly on inverters having photovoltaic generators connected thereto. A beneficial efficiency is desired for such inverters in an operating range as wide as possible under these conditions.
Also described in the prior art, photovoltaic inverters often consist of several stages, e.g., of a DC/DC converter located on the input side and of a DC-AC converter.